The present invention relates to electro-optical target recognition systems, in general, and more particularly, to a passive ranging system adapted for use in conjunction therewith to improve the performance thereof.
Electro-optical (E-O) automatic target recognition systems generally include the preprocessing, registration, and classification of E-O sensor image information of a plurality of frames common to the same field of view (FOV) or scene. In most E-O systems, the image sensor may be either a conventional television camera (TV) or a forward-looking infrared camera (FLIR). Some E-O target recognition systems may be adapted for use on board an aircraft or the like in connection with reconnaissance and/or weapon delivery tasks. In these systems, certain image features of one or more objects, preferably targets, may be extracted from the image scene of one frame for registration with extracted features of common objects of another frame viewing the same image scene. Once the common extracted object image features of one frame are registered with respect to another frame in accordance with some best-fit criteria, for example, a classification of the object in question may be performed utilizing a set of predetermined reference target image features.
However, without range information to an object as observed by an E-O imaging system, classification of the object as a target may become an onerous or somewhat confusing task especially for an automatic recognition system. For example, when an E-O sensor on board an aircraft is pointed towards the horizon, an entire clearing in the field of view thereof may appear blob-like in the sensor frame image and may possibly be characterized as a tactical target if the range to the clearing is unknown. With range information available however, such potential false alarms may be avoided merely on the basis of size of the object or area in question. In fact, target recognition tests with an E-O system have demonstrated improved performance, particularly in the rejection of false alarms, if accurate range information of the object or target in question is available.
Active ranging processors have been proposed to provide the needed range information for some airborne automatic target recognition systems. However, since these processors rely on the emissions of radiation in the form of microwave or laser energy, for example, in a hostile environment, the emitted radiation has the undesirable effect of providing an opportunity of detection and possible counterattack by an enemy. For this reason, passive ranging systems are felt safer, especially in these hostile environments, in that there is no reliance on energy radiation for range computations, and thus no transmission signal for an adversary to detect and track.
Some airborne E-O target recognition systems derive a crude estimate of range passively by combining the aircraft altitude measurements with the E-O sensor depression angle. Needless to say, the accuracy of this computation depends strongly on the assumption that the elevation of the object in question is the same as the ground reference used for the aircraft altitude measurement (i.e., flat earth concept). For all practical purposes, this approach suffers from its insensitivity to the actual terrain configurations and from its over sensitivity to errors especially at low grazing angles where the range to the target also depends heavily on the location of the target in the field of view. In some cases, at low altitudes, errors in the range measurement caused by the failure of the aforementioned assumption (flat earth) appear to mount rapidly to intolerable levels.
Apparently, a passive ranging system which can avoid the aforementioned difficulties in measuring range and provide an accurate range measurement to the object in question or to a point or area in the vicinity thereof could compliment and improve the performance of an E-O target recognition system in the automatic identification and classification of various objects in its field of view.